Real SQL Programming 1

Transcription

1 Real SQL Programming 1

2 SQL in Real Programs We have seen only how SQL is used at the generic query interface an environment where we sit at a terminal and ask queries of a database Reality is almost always different: conventional programs interacting with SQL 2

3 Options 1. Code in a specialized language is stored in the database itself (e.g., PSM, PL/pgsql) 2. SQL statements are embedded in a host language (e.g., C) 3. Connection tools are used to allow a conventional language to access a database (e.g., CLI, JDBC, PHP/DB) 3

4 Stored Procedures PSM, or persistent stored modules, allows us to store procedures as database schema elements PSM = a mixture of conventional statements (if, while, etc.) and SQL Lets us do things we cannot do in SQL alone 4

5 Procedures in PostgreSQL CREATE PROCEDURE <name> ([<arguments>]) AS $$ <program>$$ LANGUAGE <lang>; PostgreSQL only supports functions: CREATE FUNCTION <name> ([<arguments>]) RETURNS VOID AS $$ <program>$$ LANGUAGE <lang>; 5

6 Parameters for Procedures Unlike the usual name-type pairs in languages like Java, procedures use modename-type triples, where the mode can be: IN = function uses value, does not change OUT = function changes, does not use INOUT = both 6

7 Example: Stored Procedure Let s write a procedure that takes two arguments b and p, and adds a tuple to Sells(bar, beer, price) that has bar = C.Ch., beer = b, and price = p Used by Cafe Chino to add to their menu more easily 7

8 The Procedure CREATE FUNCTION ChinoMenu ( IN b CHAR(20), IN p REAL ) RETURNS VOID AS $$ INSERT INTO Sells VALUES( C.Ch., b, p); $$ LANGUAGE plpgsql; Parameters are both read-only, not changed The body --- a single insertion 8

9 Invoking Procedures Use SQL/PSM statement CALL, with the name of the desired procedure and arguments Example: CALL ChinoMenu( Eventyr, 50); Functions used in SQL expressions wherever a value of their return type is appropriate No CALL in PostgreSQL: SELECT ChinoMenu( Eventyr, 50); 9

10 Kinds of PL/pgsql statements Return statement: RETURN <expression> returns value of a function Like in Java, RETURN terminates the function execution Declare block: DECLARE <name> <type> used to declare local variables Groups of Statements: BEGIN... END Separate statements by semicolons 10

11 Kinds of PL/pgsql statements Assignment statements: <variable> := <expression>; Example: b := Od.Cl. ; Statement labels: give a statement a label by prefixing a name and a colon 11

12 IF Statements Simplest form: IF <condition> THEN <statements(s)> END IF; Add ELSE <statement(s)> if desired, as IF... THEN... ELSE... END IF; Add additional cases by ELSEIF <statements(s)>: IF THEN ELSEIF THEN ELSEIF THEN ELSE END IF; 12

13 Example: IF Let s rate bars by how many customers they have, based on Frequents(drinker,bar) <100 customers: unpopular customers: average >= 200 customers: popular Function Rate(b) rates bar b 13

14 Example: IF CREATE FUNCTION Rate (IN b CHAR(20)) RETURNS CHAR(10) AS $$ DECLARE cust INTEGER; Number of customers of bar b BEGIN cust := (SELECT COUNT(*) FROM Frequents WHERE bar = b); IF cust < 100 THEN RETURN unpopular ; ELSEIF cust < 200 THEN RETURN average ; ELSE RETURN popular ; END IF; END; Nested IF statement 14

15 Loops Basic form: <<<label>>> LOOP <statements> END LOOP; Exit from a loop by: EXIT <label> WHEN <condition> 15

16 Example: Exiting a Loop <<loop1>> LOOP... EXIT loop1 WHEN...;... END LOOP;... control winds up here If this statement is executed and the condition holds... 16

17 Other Loop Forms WHILE <condition> LOOP <statements> END LOOP; Equivalent to the following LOOP: LOOP EXIT WHEN NOT <condition>; <statements> END LOOP; 17

18 Other Loop Forms FOR <name> IN <start> TO <end> LOOP <statements> END LOOP; Equivalent to the following block: <name> := <start>; LOOP EXIT WHEN <name> > <end>; <statements> <name> := <name>+1; END LOOP; 18

19 Other Loop Forms FOR <name> IN REVERSE <start> TO <end> LOOP <statements> END LOOP; Equivalent to the following block: <name> := <start>; LOOP EXIT WHEN <name> < <end>; <statements> <name> := <name> - 1; END LOOP; 19

20 Other Loop Forms FOR <name> IN <start> TO <end> BY <step> LOOP <statements> END LOOP; Equivalent to the following block: <name> := <start>; LOOP EXIT WHEN <name> > <end>; <statements> <name> := <name>+<step>; END LOOP; 20

21 Queries General SELECT-FROM-WHERE queries are not permitted in PL/pgsql There are three ways to get the effect of a query: 1. Queries producing one value can be the expression in an assignment 2. Single-row SELECT... INTO 3. Cursors 21

22 Example: Assignment/Query Using local variable p and Sells(bar, beer, price), we can get the price Cafe Chino charges for Odense Classic by: p := (SELECT price FROM Sells WHERE bar = C.Ch AND beer = Od.Cl. ); 22

23 SELECT... INTO Another way to get the value of a query that returns one tuple is by placing INTO <variable> after the SELECT clause Example: SELECT price INTO p FROM Sells WHERE bar = C.Ch. AND beer = Od.Cl. ; 23

24 Cursors A cursor is essentially a tuple-variable that ranges over all tuples in the result of some query Declare a cursor c by: DECLARE c CURSOR FOR <query>; 24

25 Opening and Closing Cursors To use cursor c, we must issue the command: OPEN c; The query of c is evaluated, and c is set to point to the first tuple of the result When finished with c, issue command: CLOSE c; 25

26 Fetching Tuples From a Cursor To get the next tuple from cursor c, issue command: FETCH FROM c INTO x 1, x 2,,x n ; The x s are a list of variables, one for each component of the tuples referred to by c c is moved automatically to the next tuple 26

27 Breaking Cursor Loops (1) The usual way to use a cursor is to create a loop with a FETCH statement, and do something with each tuple fetched A tricky point is how we get out of the loop when the cursor has no more tuples to deliver 27

28 Breaking Cursor Loops (2) Many operations returns if a row has been found, changed, inserted, or deleted (SELECT INTO, UPDATE, INSERT, DELETE, FETCH) In plpgsql, we can get the value of the status in a variable called FOUND 28

29 Breaking Cursor Loops (3) The structure of a cursor loop is thus: <<cursorloop>> LOOP FETCH c INTO ; IF NOT FOUND THEN EXIT cursorloop; END IF; END LOOP; 29

30 Example: Cursor Let us write a procedure that examines Sells(bar, beer, price), and raises by 10 the price of all beers at Cafe Chino that are under 30 Yes, we could write this as a simple UPDATE, but the details are instructive anyway 30

31 The Needed Declarations CREATE FUNCTION RaisePrices() RETURNS VOID AS $$ DECLARE thebeer CHAR(20); theprice REAL; c CURSOR FOR (SELECT beer, price FROM Sells WHERE bar = C.Ch. ); Used to hold beer-price pairs when fetching through cursor c Returns Cafe Chino s price list 31

32 The Procedure Body BEGIN OPEN c; <<menuloop>> LOOP FETCH c INTO thebeer, theprice; EXIT menuloop WHEN NOT FOUND; IF theprice < 30 THEN UPDATE Sells SET price = theprice + 10 WHERE bar = C.Ch. AND beer = thebeer; END IF; END LOOP; CLOSE c; END;$$ LANGUAGE plpgsql; Check if the recent FETCH failed to get a tuple If Cafe Chino charges less than 30 for the beer, raise its price at at Cafe Chino by 10 32

33 Tuple-Valued Variables PL/pgsql allows a variable x to have a tuple type x R%ROWTYPE gives x the type of R s tuples R could be either a relation or a cursor x.a gives the value of the component for attribute a in the tuple x 33

34 Example: Tuple Type Repeat of RaisePrices() declarations with variable bp of type beer-price pairs CREATE FUNCTION RaisePrices() RETURNS VOID AS $$ DECLARE CURSOR c IS SELECT beer, price FROM Sells WHERE bar = C.Ch. ; bp c%rowtype; 34

35 RaisePrices() Body Using bp BEGIN OPEN c; LOOP FETCH c INTO bp; EXIT WHEN NOT FOUND; IF bp.price < 30 THEN UPDATE Sells SET price = bp.price + 10 WHERE bar = C.Ch. AND beer = bp.beer; END IF; END LOOP; CLOSE c; END; Components of bp are obtained with a dot and the attribute name 35

36 Database-Connection Libraries 36

37 Host/SQL Interfaces Via Libraries The third approach to connecting databases to conventional languages is to use library calls 1. C + CLI 2. Java + JDBC 3. PHP + PEAR/DB 37

38 Three-Tier Architecture A common environment for using a database has three tiers of processors: 1. Web servers talk to the user. 2. Application servers execute the business logic 3. Database servers get what the app servers need from the database 38

39 Example: Amazon Database holds the information about products, customers, etc. Business logic includes things like what do I do after someone clicks checkout? Answer: Show the how will you pay for this? screen 39

40 Environments, Connections, Queries The database is, in many DB-access languages, an environment Database servers maintain some number of connections, so app servers can ask queries or perform modifications The app server issues statements: queries and modifications, usually 40

41 JDBC Java Database Connectivity (JDBC) is a library similar for accessing a DBMS with Java as the host language 221 drivers available: PostgreSQL, MySQL, Oracle, ODBC,

42 Making a Connection import java.sql.*;... Loaded by forname URL of the database your name, and password go here The JDBC classes Class.forName( org.postgresql.driver ); Connection mycon =... DriverManager.getConnection( ); The driver for postgresql; others exist 42

43 URL for PostgreSQL database jdbc:postgresql://<host>[:<port>]/ <database>?user=<user>& password=<password> Alternatively use getconnection variant: getconnection( jdbc:postgresql:// <host>[:<port>]/<database>, <user>, <password>); DriverManager.getConnection( jdbc:pos tgresql:// /petersk09, petersk09, geheim ); 43

44 Statements JDBC provides two classes: 1. Statement = an object that can accept a string that is a SQL statement and can execute such a string 2. PreparedStatement = an object that has an associated SQL statement ready to execute 44

45 Creating Statements The Connection class has methods to create Statements and PreparedStatements Statement stat1 = mycon.createstatement(); PreparedStatement stat2 = mycon.createstatement( ); SELECT beer, price FROM Sells + WHERE bar = C.Ch. createstatement with no argument returns a Statement; with one argument it returns a PreparedStatement 45

46 Executing SQL Statements JDBC distinguishes queries from modifications, which it calls updates Statement and PreparedStatement each have methods executequery and executeupdate For Statements: one argument the query or modification to be executed For PreparedStatements: no argument 46

47 Example: Update stat1 is a Statement We can use it to insert a tuple as: stat1.executeupdate( INSERT INTO Sells + VALUES( C.Ch., Eventyr,30) ); 47

48 Example: Query stat2 is a PreparedStatement holding the query SELECT beer, price FROM Sells WHERE bar = C.Ch. executequery returns an object of class ResultSet we ll examine it later The query: ResultSet menu = stat2.executequery(); 48

49 Accessing the ResultSet An object of type ResultSet is something like a cursor Method next() advances the cursor to the next tuple The first time next() is applied, it gets the first tuple If there are no more tuples, next() returns the value false 49

50 Accessing Components of Tuples When a ResultSet is referring to a tuple, we can get the components of that tuple by applying certain methods to the ResultSet Method getx (i ), where X is some type, and i is the component number, returns the value of that component The value must have type X 50

51 Example: Accessing Components Menu = ResultSet for query SELECT beer, price FROM Sells WHERE bar = C.Ch. Access beer and price from each tuple by: while (menu.next()) { } thebeer = menu.getstring(1); theprice = menu.getfloat(2); /*something with thebeer and theprice*/ 51

52 Important Details Reusing a Statement object results in the ResultSet being closed Always create new Statement objects using createstatement() or explicitly close ResultSets using the close method For transactions, for the Connection con use con.setautocommit(false) and explicitly con.commit() or con.rollback() If AutoCommit is false and there is no commit, closing the connection = rollback 52

53 PHP A language to be used for actions within HTML text Indicated by <?PHP code?>. DB library exists within PEAR (PHP Extension and Application Repository) Include with include(db.php) 53

54 Variables in PHP Must begin with $ OK not to declare a type for a variable But you give a variable a value that belongs to a class, in which case, methods of that class are available to it 54

55 String Values PHP solves a very important problem for languages that commonly construct strings as values: How do I tell whether a substring needs to be interpreted as a variable and replaced by its value? PHP solution: Double quotes means replace; single quotes means do not 55

56 Example: Replace or Not? $100 = one hundred dollars ; $Peter = You owe me $100. ; $Lars = You owe me $100. ; Value of $Peter is You owe me $100, while the value of $Lars is You owe me one hundred dollars 56

57 PHP Arrays Two kinds: numeric and associative Numeric arrays are ordinary, indexed 0,1, Example: $a = array( Paul, George, John, Ringo ); Then $a[0] is Paul, $a[1] is George, and so on 57

58 Associative Arrays Elements of an associative array $a are pairs x => y, where x is a key string and y is any value If x => y is an element of $a, then $a[x] is y 58

59 Example: Associative Arrays An environment can be expressed as an associative array, e.g.: $myenv = array( phptype => pgsql, hostspec => localhost, port => 5432, database => petersk09, username => petersk09, password => geheim ); 59

60 Making a Connection With the DB library imported and the array $myenv available: $mycon = DB::connect($myEnv); Function connect in the DB library Class is Connection because it is returned by DB::connect() 60

61 Executing SQL Statements Method query applies to a Connection object It takes a string argument and returns a result Could be an error code or the relation returned by a query 61

62 Example: Executing a Query Find all the bars that sell a beer given by the variable $beer $beer = Od.Cl. ; Method application $result = $mycon->query( SELECT bar FROM Sells. WHERE beer = $beer ; ); Concatenation in PHP Remember this variable is replaced by its value. 62

63 Cursors in PHP The result of a query is the tuples returned Method fetchrow applies to the result and returns the next tuple, or FALSE if there is none 63

64 Example: Cursors while ($bar = $result->fetchrow()) { // do something with $bar } 64

65 Example: Tuple Cursors $bar = C.Ch. ; $menu = $mycon->query( SELECT beer, price FROM Sells WHERE bar = $bar ; ); while ($bp = $result->fetchrow()) { } print $bp[0]. for. $bp[1]; 65